Image sensors and other light sensitive sensors may be fabricated to detect the intensity of light received by the sensor. These sensors typically generate electronic signals that have amplitudes that are proportional to the intensity of the light received by the sensor. The sensors can convert an optical image into a set of electronic signals. The electronic signals may represent, for example, intensities of light received by the sensor. The electronic signals can also be conditioned and sampled to allow image processing.
One of the currently available types of image sensors is commonly referred to as an active pixel sensor. Active pixel sensors are typically fabricated using standard complementary metal-oxide semiconductor (CMOS) processes enabling these sensors to be integrated with digital and analog signal processing circuitry.
In conventional active pixel sensors, each pixel cell typically comprises photosensitive and non-photosensitive devices. The types of photosensitive devices include photodiodes, photoconductors, photogate MOS capacitors, and other similar devices. Non-photosensitive devices that may be found in many active pixel sensors include one or more transistors.
In many active pixel sensors, the photosensitive devices compete with non-photosensitive devices for available space on the sensor. Advances in CMOS processes permit the fabrication of sensors having pixel cells with increasing smaller geometries. As a result, the junction depth of the PN junctions and the depletion width of the MOS capacitors shrink proportionally. However, many CMOS fabricated sensors contain junction depths that are so shallow that they become much smaller than the absorption length of visible light in silicon substrate. As such, conventional active pixel sensors may suffer from deteriorating photosensitivity that may be proportional to the shrinking of the baseline CMOS process.
A variety of existing active pixel sensors are fabricated by layering a translucent conductive layer over a PIN or NIP photodiode, which is formed over a substrate. Typically, the bottom layer of the PIN or NIP photodiodes is connected to a pixel electrode that is associated with an individual pixel in the pixel cell array.
In some sensors, a voltage is applied to the top transparent conductive layer to reverse-bias the PIN (or NIP) photodiode. In conventional three-transistor pixel cells, for example, the charge collecting pixel electrode is electrically shorted with the charge-sensing node of the pixel cell. Thus, during the charge integration process, the electrical potential of the pixel electrode may vary from pixel to pixel, depending on the amount of charge collected at each pixel site. In many sensors, each of an array pixel cells may have pixel electrodes that are electrically connected because they all share a common bottom layer of the PIN or NIP diode.
A problem that typically occurs when neighboring pixel electrodes are not electrically isolated from one another is commonly referred to as pixel crosstalk. Pixel crosstalk may occur in conventional active pixel sensors when current flows from higher-potential electrodes to neighboring, lower-potential, electrodes. The presence of pixel crosstalk is often undesirable because it may result in the capturing of a blurred image.
While there have been some attempts to design sensors that alleviate or minimize undesirable affects, such as pixel crosstalk, these attempts have not been entirely successful. Accordingly, a present need exists for an active pixel sensor that can provide, for example, increasingly sharper images by minimizing undesirable affects such as pixel crosstalk.